When shielding material becomes adhered to a photomask (black defect 1) used for the production of semiconductor circuit elements or when any part of the photomask is missing (white defect 2) as shown in FIG. 2, semiconductor circuit elements cannot be produced as designed. A practical method for repairing such defects is to perform assist deposition or etching on the defects using a focused ion beam. However, the mask cannot be repaired with high precision if the position of the defect cannot be identified accurately. To achieve high precision, it would be necessary, first of all, to gain an accurate recognition of the reference position (pin point hole 5) set in the mask to minimize the difference between the contour recognition of pattern 3 and the drift, and secondly, to accurately recognize the actual contour of mask pattern 3 shaped on the substrate as a position relative to the pin point hole 5. The condition of the mask is then observed by acquiring a microscopic view of the mask using a microscope such as a scanning ion microscope. The displayed image of the pattern contour may drift significantly from the actual image, depending on which threshold value is used to recognize the boundary between the pattern contour and the substrate. FIG. 3A shows the sectional view of the boundary of pattern 3 on a glass substrate 4. FIG. 3B shows an example of these secondary electron detection signals. For example, as shown in FIG. 3B1, the pattern contour image varies considerably (between contour position a1 and a2) depending on whether the threshold value used for comparing with the secondary ion detection quantity is set to be high 11, or low 12. In the case of pattern 3, it is desirable to recognize the boundary by detecting the shoulder portion of the shielding material such as chrome. However, it is difficult to set an appropriate threshold value 11 in cases, for example, where a background component BG within the signal is large and the signal difference between the substrate area and the shielding material area is small, as shown in FIG. 3B2. In addition, since the background component is not constant and changes depending on place and time, there may be cases where the pattern cannot be recognized because the threshold value 13 is set to be higher than the pattern area signal, or on the contrary, when the glass substrate area is also recognized as a part of the pattern area because the threshold value is set to be lower than the pattern area signal, as a result of the varying influence of the background component. Moreover, since there are problems with the position of the boundary shifting in both cases where detected secondary ions are taken to be part of the substrate-side substance and cases where the secondary ions detected are taken to be part of the blocking material, detection of a shoulder portion of a blocking material such as chrome, etc. so as to confirm a boundary for precise displaying is not straightforward.
A differential processing method has been applied as a related method for emphasizing the contour to distinguish this boundary line. However, in this method, small signal fluctuations that do not correspond to the boundary may also be included as the subject of differential processing and the peak level as a result may become unnecessarily large, making after treatment troublesome. For this reason, this method cannot necessarily be considered as the most appropriate means of processing.
The object of the present invention is to solve the problems encountered in the related technology described above i.e. to provide a technology to accurately recognize and display the boundary between the substrate and the shielding material on the photomask that will in turn allow for high precision repair of defects on the photomask conducted based on accurate recognition of the position of this boundary.